Electrochemical gas sensors are well known for determining a variety of noxious gases, including hydrogen sulfide, chlorine, nitric oxide, carbon monoxide and hydrocarbons. It is now conventional to apply the active electrode material, which may be an electrocatalyst, to a membrane that is permeable to the gas to be analyzed and is impermeable to the electrolyte. An electrochemical reaction occurs at the electrode, ordinarily called the working electrode, where the electrolyte, the active electrode material and gas diffusing through the membrane interface. An opposing counter electrode is immersed in the electrolyte and the potential or current of the cell is a measure of the gas concentration exposed to the working electrode.
UK Pat. No. 2,001,763B is a galvanic electrochemical sensor for CO in which the working electrode and counter electrode are identical noble metals in contact with an acid electrolyte, the counter electrode being immersed in the electrolyte. CO, or other depolarizing species, is measured by measuring the current or voltage across the electrodes. The cell is said to be immune to zero drift.
Such galvanic cells and other potentiostated electrochemical cells are temperature sensitive due to the change in rate of the electrochemical reaction with change in temperature in accordance with the Arrhenius equation for a simple gas reaction under kinetic control: EQU K=A exp-(Ea/RT)
where K is the kinetic rate constant, A is a constant, Ea is the activation energy of the reaction, R is the gas contact and T is the absolute temperature. In practical application such as CO oxidation on platinum in sulfuric acid, this relationship is approximately correct. In a cell to sense CO in air, there is also a small oxidizing current present in the absence of any CO (zero current). The reaction or reactions causing the zero current is not totally understood but it is believed to result from oxidation of the submerged platinum electrode. Although these currents are small, they are very temperature sensitive resulting in a significant change of zero current with change in temperature. Such a zero current temperature dependence is common in electrochemical gas sensors.